The present invention concerns a signalling packet for a communication system.
In communication systems, for example the GSM pan-European digital cellular mobile radio system, a terminal and a base station communicate by means of communication channels carrying radio signals. Systems of this kind include a plurality of channels for transmission from the terminals to the base stations or for transmission from the base stations to the terminals.
These channels include a control channel that is transmitted continuously and that enables a terminal to access a system via the base station transmitting this channel in order to set up calls. The terminal must therefore identify this control channel in order to acquire the information enabling it to declare itself within the system. This information includes synchronization information and this procedure is usually therefore called the synchronization procedure.
In the solution generally adopted the synchronization procedure is carried out in two stages. Initially the terminal measures the power of all the receive channels. The terminal then attempts to synchronize to the channel it receives at the highest power; if it fails to do this, it tries the other channels in decreasing received power order, until it is eventually able to synchronize. This solution is that set out in GSM Recommendations 4.08 and 5.08.
The synchronization procedure is executed systematically when the terminal is switched on and, more generally, after any loss of synchronization, i.e. if the radio link carried by the control channel between the base station and the terminal is interrupted. This may be intentional, for example if the terminal is switched off, or unintentional. The terminal may be temporarily unable to synchronize because radio reception conditions are inadequate. This occurs in a tunnel, for example, or more generally when it is in a shadow area in the radio sense of this term.
Synchronization usually comprises two phases. The first or frequency synchronization phase consists in acquiring the frequency reference of the base station. The second or time synchronization phase consists in acquiring the time reference of the base station.
To this end, in the GSM system the control channel BCCH includes two sub-channels, namely a frequency control sub-channel FCH for the frequency synchronization and a synchronization sub-channel SCH for the time synchronization.
The frequency control sub-channel is in the form of a packet corresponding to a pure sinusoid transmitted at regular time intervals. The terminal must therefore look for this packet for a time period that substantially corresponds to the packet repetition period.
Because of the nature of the packet and the means employed to detect it, it is not possible to determine its start or its duration, and it is therefore necessary to use the synchronization sub-channel to achieve the necessary time synchronization.
The synchronization sub-channel follows the frequency control sub-channel with a known time-delay. It comprises a sequence of symbols having a suitable autocorrelation function. The terminal knows this synchronization sequence and correlates this sequence with the sequence of symbols received. Because of the inaccuracy of the time reference of the base station, the terminal does not know with certainty which of the symbols received corresponds to the first symbol of the synchronization sequence and many correlations are therefore required, shifting one of the sequences relative to the other, in order to identify the correlation peak.
A first aim of the invention is therefore to improve the performance of the synchronization procedure.
In most radio communication systems the radio signal transmission subsystem includes an element that is mobile. As a result, the frequency of the radio signal is modified because of the Doppler effect.
A Doppler shift naturally occurs in the GSM system if the terminal is moving, and its value is directly proportional to the speed of the terminal. Although it is relatively easy to detect the pure sinusoid of the frequency control sub-channel when the latter has a known frequency, for example using a selective filter, this is no longer so if the frequency is modified by an unknown Doppler shift. A filter is then required with a bandwidth that allows for the maximum Doppler shift in both directions, which seriously degrades the performance of the filter.
What is more, the frequency shift is much greater in systems using non-geostationary satellites, as in the Globalstar system in particular.
A system of this kind uses a satellite in low Earth orbit, for example at an altitude of 1 390 kilometers, travelling at a speed of for example around 7.2 kilometers per second, as a relay station between the terminal and the base station.
The satellite receives the radio signal from the base station and retransmits it to the terminal. The satellite simply acts as a "mirror": it transmits the signal it receives from the base station without modification, or at most after transposing its frequency.
The frequency shift depends on the speed and on the position of the satellite relative to the terminal, which can itself be regarded as fixed.
It is clear that this frequency shift must be corrected.
Another aim of the invention is therefore to provide means for correcting the frequency shift due to the Doppler effect.
In radio communication systems the communication channels proper are often transmitted as follows:
A transmitter transmits a sequence of symbols to a receiver on a transmission channel. The sequence transmitted is degraded in the transmission channel with the result that the sequence of symbols received by the receiver is not identical to it. The main deterioration is intersymbol interference due to the fact that a symbol can take different paths in the transmission channel. If at least two paths have a path difference greater than the distance between two symbols transmitted successively, a symbol taking one of these paths will interfere with a subsequent symbol taking another, shorter path.
An equalizer is used in the receiver to correct the intersymbol interference. To operate correctly, the equalizer must know the impulse response of the transmission channel. To this end, known symbols are transmitted in a training sequence. The training sequence is chosen to suit the characteristics of the transmission channel and in particular its length.
Given that the symbols are transmitted regularly, with a period called the symbol period, the length of the channel is defined as the number of symbol periods that is equivalent to the difference between the longest path and the shortest path of the channel.
A channel estimator device is used in the receiver to establish the impulse response of the channel. It generates a replica of the training sequence and correlates it with the sequence of symbols received. The result is a set of coefficients h.sub.i in which i varies from 0 to L, where L is the length of the channel, this set of coefficients providing information used by the equalizer. The most direct path on the channel is represented by ho and the other coefficients represent longer paths that interfere with it.
It is clear that the channel cannot be estimated until synchronization has been acquired. Moreover, a special packet, namely the training sequence, must be provided for this purpose.
A third aim of the invention is therefore to provide channel estimation means.
To summarize, in any communication system many signalling signals are required to set up a call. In the present context the term "synchronization" is to be understood in the most general sense and in particular it covers synchronization information, information needed to correct the Doppler shift and information that is used to estimate the transmission channel.